MOBT-689 (Pt 1): Move functionality out of the spot-extract CLI into a plugin

[bayliffe]

Prior to adding new functionality to the spot-extract plugins it was necessary to simplify the CLI inline with the changes that are needed for implementing IMPROVER configurations using DAGRunner. These changes are:

• Move the logic from the CLI to a new plugin (SpotManipulation, hate the name and open to suggestions).
• Add unit tests to cover this logic which was previously only checked with acceptance tests.
• Make the neighbour_finding_method_name functionality a utility as it is used in several places. This includes the creation of a new unit test file for these spot-data utilities.
• Move the check_grid_match utility from the spotdata module to the metadata module where the related functionality lives.

Pt 2 of this work will add the functionality to extract a subset of spot sites from an existing spot-forecast cube using the spot-extract CLI / SpotManipulation plugin. This was large enough so I've stopped here to split it up, though Pt 2 should be much smaller.

Testing:

• [x] Ran tests and they passed OK
• [x] Added new tests for the new feature(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.41%. Comparing base (84a8944) to head (839a422). Report is 8 commits behind head on master.

:exclamation: Current head 839a422 differs from pull request most recent head b9c944e

Please upload reports for the commit b9c944e to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1996 +/- ## ========================================== + Coverage 98.39% 98.41% +0.02% ========================================== Files 124 126 +2 Lines 12212 12402 +190 ========================================== + Hits 12016 12206 +190 Misses 196 196 ```

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